The present invention relates to a method of scrolling a figure, such as a map on a display screen.
In a system in which a figure, such as a topographic map, a street map or the like, is stored as electrical information and is graphically displayed on a display device such as a CRT, it is known to continuously move a domain of the figure for display. Such an operation is called scrolling.
A system for scrolling a figure has been described by, for example, COMPUTER GRAPHICS written by J. D. Forly & A. VAN DAM and translated by Atsuyoshi IMAMIYA. In the described system, scrolling is carried out in such a manner that graphic data is stored in a frame memory of a graphic display device each time a portion of the figure to be displayed (or a domain of the figure for display) is changed, and the access to the frame memory attendant upon refreshing and the refreshing of a display screen or a graphic image are repeated.
Also, JP-A-62-180473 has disclosed a system in which scrolling is carried out in such a manner that a frame memory having a large capacity is provided and the whole of a certain picture is collectively stored in the large capacity frame memory even if it comprises a plurality of graphic files. It is a characteristic of this system that it permits scrolling in a way which is transparent to the limit of each figure area to a certain extent.
Further, JP-A-1-62769 has disclosed a system in which the amount of a figure as displayed is controlled by determining the number of hierachical levels of graphic data to be displayed in accordance with the data amount of graphic data which is present in a designated display domain. It is a characteristic of this system that it permits high-speed scrolling.
In each of the above-mentioned scrolling systems, a figure developed in the frame memory is displayed and scrolled. Accordingly, as the amount of data which comprises figure is increased, the time required for refreshing becomes long. In the system disclosed in JP-A-1-62769, the time for access to the frame memory is reduced by decreasing the number of figures to be displayed. Even in this system, however, the amount of data for a figure to be displayed becomes large when the domain of the figure for display is enlarged. This can be avoided by further decreasing the number of figures to be displayed. In that case, however, there arises an inconvenience that a figure the display of which is desired may be erased. Further, in this type of system, though it is important to provide the ability to effect high-speed scrolling, it is desired to make a graphic image upon scrolling and before and after scrolling easy for an operator to see. Also, it is desired to improve the operability upon scrolling.